A known contact device of this type is described in European patent publication EP-A-02 15146. In this system, the contact needles at their front ends, i.e. the ends which are to contact the test article, are guided in bores of a front plate of the plate assembly. The bores of the front plate are distributed in accordance with the test points on the article at which the contact needles are to engage the latter.
The contact needles, in the region of their rear ends, are held by an elastic plate through which the needles pass so that the needles cannot fall out of the test adapter.
At their rear ends, which are formed with a frustoconical configuration, these needles engage in conical bores of the heads of the spring-loaded countercontact elements to establish electrical connection therewith.
The frustoconical rearwardly converging tips of the contact needles are affected by dirt and other contaminants, such as abrasion products and wear products of the contact device between the frustoconical tips and the frustoconical sockets of the countercontact elements, which contaminants pass between the two members which are to make electrical contact. In practice, this is observable as a sharp increase in the electrical resistance of the conductive path between the countercontact element and the contact needle or even in interruption in the electrical continuity therebetween.
This type of connection between this contact needle and the countercontact element having respectively a frustoconical rearward tip and a conical bore receiving the tip cannot prevent sudden and unpredictable contamination and thus uncontrolled electrical resistances or unexpected electrical path interruptions from developing.
While uncontrolled increases in electrical resistance can result in improper readings and measurements which may signal faults in the test article where none may exist, interruptions in continuity can render the entire test device unusable.
It has also been found that the retention of the contact needles in an elastic plate with extremely small pitch rasters, i.e. with a spacing between nearest neighbors of the needles of say 1/10th of an inch or often substantially less contributes the possibility of problems since the elastic plate does not permit independent resilient displacement of each needle. By and large it is found that with such elastic means practically a constant displacement is necessary for all needles to ensure effective resilient support for the needles. In practice, however, the contact needles may have to engage points of different heights on the test article so that there are differences in the resilient or spring force applied to the contact needles.